1. Field of the Invention
The present invention relates to an Snxe2x80x94Bi alloy plating bath, and more particularly to an Sn-Bi alloy plating bath which does not erode a piece to be plated, and has a high stability.
2. Description of the Related Art
In the electronic industrial field, Snxe2x80x94Pb alloy platings have been widely used for soldering electrodes. In recent years, there have been anxieties about the influence of the Pb contained in the Snxe2x80x94Pb alloy platings which may be exerted over the environment. Sn alloy platings which to not contain Pb have been demanded. Therefore, greater attention has been paid to Snxe2x80x94Bi alloy platings, which have a low melting point and excellent soldering properties.
Many of Snxe2x80x94Bi alloy plating baths have a strong acidity, namely, pH 1.0 or lower, in order to dissolve large amounts of bismuth. Since a large part of the electronic components pieces to be plated are composites containing ceramics, glass, ferrite, and so forth, there has been the problem that the electronic components become eroded by such high strong acidic baths, causing the deterioration of their characteristics.
For the purpose of improving the problem of the eroding properties, Japanese Unexamined Patent Publication No. 6-340994 and Japanese Unexamined Patent Publication No. 7-138782 disclose Snxe2x80x94Bi alloy plating baths containing various complexing agents and having a pH of 2.0-9.0. Bismuth ions and tin ions are stabilized in the baths by addition of the complexing agents. As a result, plating baths within the range of from weak acidity to neutral are realized. However, these plating baths have problems of stability, and should be improved further to be used industrially.
The present invention is directed to a Snxe2x80x94Bi alloy plating bath which is stable enough to use continuously in the electronic industrial field and a method of plating using the Snxe2x80x94Bi alloy plating bath. The Snxe2x80x94Bi alloy plating bath has a pH of about 2.0 to 9.0 and comprises Bi3+ ions, Sn2+ ions, a complexing agent (I) and a complexing agent (II).
The complexing agent (I) is selected from the group consisting of (a) aliphatic dicarboxylic acids having alkyl groups of 1-3 carbon atoms, (b) aliphatic hydroxymonocarboxylic acids having alkyl groups of 1-3 carbon atoms, (c) aliphatic hydroxypolycarboxylic acids having alkyl groups of 1-4 carbon atoms, (d) monosaccharides, polyhydroxycarboxylic acids produced by partially oxidizing the monosaccharides, and their cyclic ester compounds, and (e) condensed phosphoric acids.
The complexing agent (II) is selected from the group consisting of (s) ethylenediaminetetraacetic acid (EDTA), (t) nitrilotriacetic acid (NTA), and (u) trans-1,2-cyclohexanediaminetetraacetic acid (CyDTA).
The Snxe2x80x94Bi alloy plating bath has a pH of about 2.0 to 9.0 and comprises Bi3+ ions, Sn2+ ions, complexing agent (I) and complexing agent (II).
Preferably, the concentration ratio of complexing agent (II) in mol/l to the Bi3+ ions in mol/l is at least about 10, the concentration ratio of complexing agent (II) in mol/l to the Sn2+ ions in mol/l is at least about 1, and the concentration ratio of complexing agent (I) in mol/l to the Sn2+ ions in mol/l is at least about 0.1.
According to the present invention, electronic components pieces made of ceramics, glass, ferrite or the like, can be plated at a high cathode current density without eroding the electronic components. The plating bath of the present invention has a high bath stability and can be used for a long time without the bath decomposition occurring.
As a result of the intensive examination by the inventors of the present invention, it has been found that the stability of a bath in the weak acidic range can be remarkably enhanced by adding to the bath a complexing agent (I) selected from the below-described (a) through (e) and a complexing agent (II) selected from the aminocarboxlic acids of the below-described (s) through (u) as complexing agents for the plating bath. The practical application of the Snxe2x80x94Bi alloy plating bath enables avoidance of eroding electronic component pieces to be plated made of ceramics, glass, ferrite or the like, and it can be used at a relatively high cathode current density and has an excellent bath-stability.
For complexing agent (I), (a) aliphatic dicarboxylic acids having alkyl groups of 1-3 carbon atoms, (b) aliphatic hydroxymonocarboxylic acids having alkyl groups of 1-3 carbon atoms, (c) aliphatic hydroxypolycarboxylic acids having alkyl groups of 1-4 carbon atoms, (d) monosaccharides, polyhydroxycarboxylic acids produced by partially oxidizing the monosaccharides, and their cyclic ester compounds, and (e) condensed phosphoric acids can be employed.
For complexing agent (II), (s) ethylenediaminetetraacetic acid (EDTA), (t) nitrilotriacetic acid (NTA), and (u) trans-1,2-cyclohexanediaminetetraacetic acid (CyDTA) can be used.
Preferable examples of (a)-(e) as the complexing agent (I) are described below. As the aliphatic dicarboxylic acids (a) having alkyl groups of 1-3 carbon atoms, malonic acid, succinic acid or the like; as the aliphatic hydroxymonocarboxylic acids (b) having alkyl groups of 1-3 carbon atoms, glycolic acid, lactic acid or the like; as the aliphatic hydroxypolycarboxylic acids (c) having alkyl groups 1-4 carbon atoms, citric acid, tartaric acid, malic acid or the like; as the monosaccharides, the polyhydroxycarboxylic acids produced by partially oxidizing the monosaccharide, and their cyclic ester compounds (d), gluconic acid, glcoheptic acid, xcex4-gluconic lactone, or the like; and as the condensed phosphoric acids (e), pyrophosphoric acid, tripolyphosphoric acid or the like, are exemplified.
In the present plating bath, preferably, the concentration ratio of complexing agent (II) (mol/l)/Bi3+ (mol/l) is at least about 10, the concentration ratio of complexing agent (II) (mol/l)/Sn2+ (mol/l) is at least about 1, and the concentration ratio of complexing agent (I) (mol/l)/Sn2+(mol/l) is at least about 0.1. With the above-described concentration ratios, a plating bath which has a high bath stability and which can be used at a high current density can be realized.
The standard electrode potential (Bi3+/Bi E0=+0.215 V) based on the standard hydrogen electrode with respect of the oxidation of Bi, is nobler than the standard electrode potential (Sn4+/Sn2+E0=0.154 V) based on the standard hydrogen electrode at which Sn2+ is oxidized to Sn4+. Therefore, in a Snxe2x80x94Bi alloy plating bath, Bi3+ is reduced with Sn2+ so that the decomposition of the bath, such as the deposition of Bi, readily occurs. Accordingly, it is important for the purpose of stabilizing the bath to select the kinds and ratios of complex ions to be in the bath. The order of magnitude of the complex stability constants between Sn and Bi with the complexing agent (I) and the complexing agent (II) used in this invention is
complexing agent (II)xe2x80x94Bi greater than  complexing agent (II)xe2x80x94Sn greater than  greater than 
complexing agent(1)xe2x80x94Bi greater than  complexing agent (I)xe2x80x94Sn.
The ratios of the respective complex ions to be produced in the bath are determined by this relationship between the magnitudes of the complex stability constants, and the concentration ratios of the respective metals to the complexing agents. A complex having a higher complex stability constant is formed precedently, and the formed complex has a higher stability.
In the composition of the plating bath of the present invention, substantially the total amount of Bi3+ forms a complex with complexing agent (II) precedently. The complexing agent (II) remaining, not coordinated to Bi3+, then forms a complex with Sn2+. The Sn2+ remaining and not forming the complex with complexing agent (II) then produces a complex with the complexing agent (I). Accordingly, three kinds of complexes, namely, the complexes of complexing agent (II) with Bi, complexing agent (II) with Sn, and complexing agent (I) with Sn, are mainly formed. The complex of complexing agent (II)xe2x80x94Bi has a very high stability since the complexing agent (II) has a much higher complexing power as compared with complexing agent (I). Therefore, Bi3+ can be prevented from being reduced with Sn2+, that is, the decomposition of the bath can be prevented.
If complexing agent (II) only is used in this plating bath, without complexing agent (I), a large part of the complexes of complexing agent (II) with Sn and complexing agent (II) with Bi are deposited as the complex salts, since the solubilities are low. Accordingly, the metal ion concentrations in the bath can not be increased, and thereby, it is difficult to use the bath at a high current density. On the other hand, by using complexing agent (I) together with complexing agent (II) as in the present invention, the solubilities of the complexes of complexing agent (II) with Sn and complexing agent (II) with Bi are enhanced. As a result, the metal ion concentrations in the bath can be increased so that the bath can be used at a high current density.
For the above-described reasons, the preferable concentration ratio of complexing agent (II) (mol/l)/Bi3+ (mol/l) is at least about 10, the concentration ratio of complexing agent (II) (mol/l)/Sn2+ (mol/l) is at least about 1, and the concentration ratio of complexing agent (I) (mol/l)/Sn2+ (mol/l) is at least about 0.1, in the plating bath of the present invention. When the concentration ratio of complexing agent (II) (mol/l)/Bi3+ (mol/l) is less than about 10, the required amount of the Bi salt can not be dissolved since the solubility of the Bi salt is low, and moreover, the complex of complexing agent (II)xe2x80x94Bi can not be formed with stability, that is, the stability of the bath can not be attained. Further, when the concentration ratio of complexing agent (II) (mol/l)/Sn2+ (mol/l) is less than about 1, the ratio of the complex of complexing agent (I)xe2x80x94Sn having a low stability is increased and the stability of the bath can not be attained. Moreover, when the concentration ratio of complexing agent (I) (mol/l)/Sn2+ (mol/l) is less than about 0. 1, the metal ion concentrations in the bath can not be increased since the solubilities of the complexities of complexing agent (II)xe2x80x94Sn and complexing agent (II)xe2x80x94Bi are low. Therefore, it is difficult to use the bath at a high current density. As regards the metal ion concentrations of the plating bath, the concentration of Sn2+ is about 0.1-0.5 (mol/l), preferably about 0.2-0.4 (mol/l), and that of Bi3+ is about 0.005-0.2 (mol/l), preferably about 0.01-0.1 (mol/l).
The pH of the Snxe2x80x94Bi alloy plating bath of the present invention is preferably about 2.0-9.0. The reason is that when the pH is less than about 2.0, the acidity is extremely strong so that electronic components made of ceramics, glass, ferrite or the like as pieces to be plated are eroded. When the pH is higher than about 9.0, the stability of the complexes is reduced. Therefore, the stability of the bath is deteriorated and the eroding properties for the electronic components are increased.
As a supply source of Sn2+, publicly known ones can be used in this invention. For example, tin sulfate, tin chloride, tin sulfamate, tin methansulfonate, tin oxide, tin hydroxide or the like, alone or in mixtures, may be used. As a supply source of Bi3+, publicly known ones may be used. For example, bismuth sulfate, bismuth chloride, bismuth sulfamate, bismuth methansulfonate, bismuth oxide, bismuth hydroxide or the like may be added solely or mixed appropriately. For the complexing agent (I) ion and the complexing agent (II) ion, their publicly known supply sources can be used, respectively. Acids, alkali metal salts ammonium salts, divalent tin salts, trivalent bismuth salts, or the like, may be added solely or mixed appropriately. When a divalent tin salt and a trivalent bismuth salt are supplied with complexing agent (I) ion and/or complexing agent (II) ion, Sn2+ and Bi3+ which are the pair ions for complexing agent (I) ion and complexing agent (II) ions, respectively, constitute a part of the concentrations of Sn2+ and Bi3+, respectively, and are included with respect to the above-described amount of the metal ions.
Further, a conductive salt may be added to the plating bath of the present invention to improve the conductivity of the plating bath. As the conductive salt, publicly known salts may be used. For example, potassium chloride, ammonium chloride, ammonium sulfate or the like may be added solely or as a mixture. Further, a pH buffer may be added to the plating bath of the present invention to reduce the variation of the pH of the bath. As the pH buffer, publicly known ones may be used. For example, the alkali metal salts, the ammonium salts or the like of boric acid and phosphoric acid may be added solely or appropriately mixed. A brightener may be added to the plating bath of the present invention in addition to the above-described components. As the brightener, nonionic surfactants such as polyoxyethylenealkylamines, alkylnaphthols or the like, amphoteric surfactants such as lauryldimethylaminoacetic acid betaine, imidazolinium betaine or the like, and cationic surfactants such as dodecyltrimethylammonium salt, hexadodecylpyridinium salt, or the like, and anionic surfactants such as polyoxyethylene alkylether sulfates, alkylbenzenesulfonates or the like may be used. In order to prevent the oxidation of Sn2+, an anti-oxidant may be added. As the anti-oxidant, publicly known ones may be used. For example, hydroquinone, ascorbic acid, catechol, resorcin or the like may be added.
The Snxe2x80x94Bi alloy plating bath of the present invention can be advantageously applied when electronic components such as chip capacitors, chip resistors, chip coils or the like are plated. As the anode, for example, Sn metal, Bi metal, Snxe2x80x94Bi alloys, titanium or carbon plated with platinum, or the like may be used. The bath temperature is about 10-50xc2x0 C., preferably about 25-30xc2x0 C. The cathode current density is about 0.1-3.0 A/dm2.